As one method for working a steel material into shapes of gears and other parts, hot forging of forging a steel material at a high temperature is widely used.
However, in hot forging, it is required to heat a material to be forged at a high temperature and also required to heat a die. Thus, there is a problem of an increase in required costs since a large amount of thermal energy is consumed and the number of working steps is increased.
The working method has been changed from hot forging to cold forging. In cold forging, heating is not required and the number of working steps is decreased. Thus, it is possible to reduce required costs.
However, since the deformation resistance of a material to be forged increases in cold forging as compared to hot forging, there are significant problems of abrasion or cracking of a die.
In order to solve this problem, a countermeasure of reducing deformation resistance at the time of forging working by decreasing the hardness of a material (steel material) through inhibition of addition of an alloy element, and the like have been conventionally taken.
However, reduction in hardness and deformation resistance through reduction of addition of an alloy element causes a decrease in the strength of parts.
Incidentally, for example, gears are produced for using, by forming steel into shapes of parts normally by cutting, forging, and the like, then subjecting the steel to carburizing and quenching for improving abrasion resistance and fatigue strength, and subjecting the steel to a surface treatment.
Conventionally, as quenching at the time of carburization, oil quenching has been a main stream. In recent years, however, quenching by gas cooling in a small carburizing furnace has been performed in some cases. Quenching by gas cooling is advantageous in that the amount of distortion at the time of gas quenching is smaller than the amount of distortion at the time of conventional oil quenching.
However, since the cooling rate of gas cooling is lower than the cooling rate of oil cooling, in order to obtain hardness that can ensure strength, it is required to add a large amount of an alloy element. When a large amount of an alloy element is added to improve hardenability, the hardness of the steel material increases and thus the aforementioned cold forgeability is deteriorated.
That is, cold forgeability and hardenability are conventionally in a trade-off relationship and there are technical difficulties in attaining both properties.
In addition, Patent Document 1 below discloses the invention relating to a “case hardening steel material having excellent cold workability and coarse grain preventing properties at the time of carburization and method for producing the same” and it is disclosed therein that the amount of Cr is limited to 1.25% or less to ensure cold workability, B is added to ensure hardenability, and for the purpose of preventing abnormal grain growth of crystal grains, the amount of precipitate particles of Ti and Nb is defined such that the density of precipitate particles of TiC and NbC having a diameter of 0.2 μm or less is 10 particles/μm2 or more.
Patent Document 1 is different from the present invention in that a large amount of Ti is added to precipitate TiC (a balance between Ti and N is different from that of the present invention), and Nb which is considered as an impurity component in the present invention is added.